1. Field of Invention
The present invention relates to a surface light source device of side light type, in particular to a surface light source device of side light type in which quality of output illumination light is improved by eliminating reduction in luminance in areas both side portion of a light guide plate. The present invention is applicable to back lighting of a liquid crystal display, for example.
2. Related Art
A surface light source device of side light type has conventionally been applied to, for example, a liquid crystal display for illuminating a liquid crystal panel from the back thereof. This arrangement is suitable for reducing thickness over all.
The surface light source device of side light type usually employs a rod-like light source such as a cold cathode tube as a primary light source arranged beside a guide plate (plate-like guide body). Illumination light emitted from the primary light source is introduced into the guide plate through a side end surface (incidence end surface) of the guide plate. Illumination light thus introduced propagates through the guide plate while being emitted toward the liquid crystal panel from one (emission surface) of major surfaces of the guide plate.
Guide plates employable for surface light source devices of side are of two types, a type having a generally uniform thickness and another type having thickness decreasing gradually away from the primary light source. Generally, the latter emits illumination light more efficiently than the former.
FIG. 6 is an exploded perspective view showing a conventional surface light source device of side light type employing the guide plate of the latter type. FIG. 7 shows a cross section taken in line A--A in FIG. 6. Referring to FIGS. 6 and 7, a surface light source device of side light type 1 comprises a guide plate 2, a primary light source 3, a reflection sheet 4, and a prism sheet 5 serving as a light control member. The reflection sheet 4, the guide plate 3 and the prism sheet 5 are laminatedly arranged. The primary light source 3 is arranged beside the guide plate 2.
The guide plate 2 is made of a light scattering guide body having a wedge-shaped cross section and is called light scattering guide plate. The light scattering guide body includes, for example, a matrix of PMMA (polymethyl methacrylate) and a multiplicity of translucent fine particles dispersed uniformly in the matrix. Refractive index of the fine particles is different from that of the matrix.
The primary light source 3 includes a cold cathode tube (fluorescent lamp) 7 and a reflector 8 having a generally semicircular cross section arranged on the back surface thereof. The reflector 8 includes a regular or irregular reflection sheet. Illumination light is supplied toward an incidence end surface 2A constituting a side end surface of the light scattering guide plate 2 through an opening of the reflector 8. The reflection sheet 4 consists of a sheet-like regular reflection member made of a metal foil or the like or a sheet-like irregular reflection member made of a white PET film or the like.
Illumination light is introduced into the guide plate 2 through the incidence end surface 2A, and while being reflected between two major surfaces (back surface 2B and emission surface 2C), propagates toward the distal end.
In the meantime, illumination light is scattered by the fine particles in the guide plate 2. In a case where the reflection sheet 4 made of the irregular reflection member is employed, illumination light is also irregularly reflected by the reflection sheet 4.
Incidence angle of illumination light to the emission surface 2C gradually decreases according to repeated reflections by the inclined back surface 2B. Such decrease of incidence angle increases components having a critical angle or less with respect to the emission surface and thus promotes emission from the emission surface. As a result, lack of emission light in an area far from the primary light source 3 is prevented.
Illumination light emitted from the emission surface 2C, which has experienced light scattering due to the fine particles in the light scattering guide plate 2 or irregular reflection on the reflection sheet 4 in addition thereto, has property of scattered light. The main direction of propagation of illumination light from the emission surface 2C, however, inclines toward a distal end with respect to the frontal direction (in direction away from the incidence end surface 2A) regarding in a plane row perpendicular to the incidence end surface 2A. That is, the light emitted from the light scattering guide plate 2 has a directivity. This property is called emission directivity.
If the back surface 2B is a smooth surface, direction of propagation of illumination light from the emission surface 2C expands toward two sides symmetric about the frontal direction regarding in a plane row parallel to the incidence end surface 2A. That is, when viewed from the incidence end surface 2A, illumination light contains components emitted obliquely rightward and leftward. In order to correct such components directionally and increase emission in the frontal direction, a prism surface (light control surface) is formed also on the back surface 2B. This prism surface has a great number of parallel prism rows. These prism rows run generally at right angles to the incidence end surface 2A. As indicated by arrow B, each prism row is configured of a minute projection having a triangular cross section, for example. Slopes 2E, 2F of these projections correct direction of light propagation so as to increase emission toward the frontal direction regarding in a plane row parallel to the incidence end surface 2A.
The prism sheet 5 arranged along the emission surface 2C is made of a translucent sheet material of polycarbonate or the like. The prism sheet 5 has a prism surface formed with a great number of parallel prism rows. In this example, the prism sheet 5 faces toward the guide plate 2 and the prism rows are oriented so as to extend generally in parallel to the incidence end surface 2A.
As indicated by arrow C, each prism row is configured of a minute projection having a triangular cross section, for example. Slopes 5A, 5B of these projections correct the obliquely-emitted illumination light in the frontal direction regarding in a plane row perpendicular to the incidence end surface 2A. The above-mentioned related art, however, has a problem yet to be solved. That is, as shown in FIG. 8, in the conventional surface light source device described above, a central area AR1 of the emission surface provides a high luminance, whereas an areas AR2 near both side portions provide a lower luminance level. The areas AR2 appear darker than the area AR1. This reduced luminance tends to be more conspicuous for a larger screen size (or a larger size of the emission surface 2C) of the display for which the surface light source device is applied.